How do the Mackerel and the mackerel-agent communicate? mackerel -agent communicates with the Mackerel via its HTTP REST API. No direct communication is done from the Mackerel side to the host that contains mackerel-agent. Encrypted with SSL/TLS correspondence. Related articles What is Mackerel's IP address and port number? What environments are supported by Mackerel? What is the source IP address for Webhook and other services’ alert notifications from Mackerel? What is the time interval between alert checks? The agent won’t startup.